The purpose of these experiments is to determine the mechanisms controlling bronchial blood flow in chloralose anesthetized sheep. In the sheep, the carinal artery is the major source of blood flow to all the intrapulmonary bronchi except those in the right apical lobe. Anatomical studies have shown that there are collateral sources of blood flow to the carinal artery bed. It has also been shown that the bronchial circulation does not show a true reactive hyperemia which may be related to the presence of collaterals or it may indicate that autoregulation of blood flow does not occur in the bronchial circulation. In this study, collateral flow will be determined as the difference of total bronchial blood flow to the carinal artery bed minus the simultaneously measured carinal artery blood flow. Autoregulation of bronchial blood flow will be studied by determining whether carinal artery flow is maintained constant as aortic pressure is varied from 50 to 200 mm Hg. Because blood flow in most systemic beds is also controlled by reflex mechanisms, the response of carinal artery flow to carotid sinus pressure will be determined before and after denervation of the aortic and carotid sinus baroreceptors. The direct and reflex responses of the bronchial circulation to chnges in arterial blood gas composition will be determined by observing the responses to ventilation with hypoxic gas mixtures before and after denervation of the aortic and carotid body chemoreceptors. The responses of the bronchial circulation after denervation will be compared with those of the pulmonary circulation as well as with other systemic beds. The direct effect of hypoxia on the pulmonary circulation is a vasoconstriction which is in contrast to the vasodilation seen in systemic beds. This comparison will be made because the bronchial circulation is located within the lung and hence may have local control mechanisms similar to the pulmonary circulation.